As is known, systems for harvesting energy (also known as “energy-scavenging systems”) from intermittent environmental-energy sources (i.e., ones that supply energy in an irregular way) have aroused and continue to arouse considerable interest in a wide range of technological fields. Typically, energy harvesting systems are configured to harvest, store, and transfer energy generated by mechanical or thermal sources to a generic load of an electrical type.
Low-frequency vibrations, such as, for example, mechanical vibrations of disturbance in systems with moving parts may be a valid source of energy. The mechanical energy is converted, by one or more appropriate transducers (for example, piezoelectric or electromagnetic devices) into electrical energy, which may be used for supplying an electrical load. In this way, the electrical load does not require batteries or other supply systems, which are cumbersome and present a low resistance in regard to mechanical stresses.
FIG. 1 is a schematic illustration in the form of functional blocks of an energy harvesting system of a known type.
FIG. 2 shows, according to a simplified circuit representation, the energy harvesting system of FIG. 1.
The energy harvesting system of FIG. 1 comprises: a transducer 2, for example of an electromagnetic or piezoelectric type, subject in use to environmental mechanical vibrations and configured to convert mechanical energy into electrical energy, typically into AC (alternating current) voltages; a scavenging interface 4, for example comprising a diode-bridge rectifier circuit (also known as Graetz bridge), configured to receive at input the AC signal generated by the transducer 2 and supplying at output a DC (direct current) signal for charging a capacitor 5 connected to the output of the rectifier circuit 4; and a DC-DC converter 6, connected to the capacitor 5 for receiving at input the electrical energy stored by the capacitor 5 and supplying it to an electrical load 8. Thus, the capacitor 5 has the function of an element for storing energy, which is made available, when required, to the electrical load 8 for operation of the latter.
This type of interface, which operates as a peak detector, presents some drawbacks. The efficiency of the system 1 of FIG. 1 is markedly dependent upon the signal generated by the transducer 2. In the absence of the DC-DC converter 6, the efficiency rapidly drops to zero (i.e., the system 1 is unable to harvest environmental energy) when the amplitude of the signal of the transducer 2 (signal VTRANSD) assumes a value lower, in absolute value, than VOUT+2VTH_D where VOUT is the voltage across the capacitor 5, and VTH_D is the threshold voltage of the diodes that form the energy harvesting interface 4. As a consequence of this, the maximum energy that may be stored in the capacitor 5 is limited to the value Emax=0.5·COUT·(VTRANSDMAX−2VTH_D)2. If the amplitude of the signal VTRANSD of the transducer 2 is lower than twice the threshold voltage VTH_D of the diodes of the rectifier of the energy harvesting interface 4 (i.e., VTRANSD<2VTH_D), then the efficiency of the system 1 is zero, the voltage across the output capacitor 5 is zero, the environmental energy is not harvested, and the electrical load 8 is not supplied.
When the DC-DC converter 6 (of a boost type) is set between the output capacitor 5 and the electrical load 8, it is possible to make up for the drop in efficiency. However, in this situation, the current supplied by the transducer and rectified by the diode bridge is not regulated and is not actively controlled. Consequently, the impedance RLOAD represented schematically in FIG. 2 cannot be matched to the series impedance RS of the transducer 2. This in any case causes a global loss of efficiency of the system 1.
A further solution, which enables active control of the current supplied by the transducer 2, envisages use of an AC-DC converter. This solution, for example proposed by IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol. 25, No. 8, August 2010, pp. 2188-2199 (incorporated by reference), envisages the use of a closed-loop boost converter that exploits directly the series inductance of the transducer and generates a regulated voltage that charges the output capacitor. It is thus possible to supply the electrical load 8 directly by the output capacitor, without the aid of a DC-DC converter 6 of the type illustrated in FIG. 1. The control loop enables the voltage on the output capacitor for being kept substantially constant. However, this solution presents some disadvantages. For instance, if the load requires a supply power that exceeds the maximum power that the output capacitor may supply, the regulated output voltage drops to zero in a substantially immediate way. Furthermore, in this condition, the AC-DC converter is unable to make up immediately for the voltage drop on the output capacitor, harvesting further energy for supply of the electrical load. The energy harvesting efficiency is thus jeopardized.